1. Field of the Invention
The present invention relates to a process for the polymerization or copolymerization of ethylene in the presence of a highly active novel catalyst, and particularly to a process for the production of a granular ethylene polymer having a large particle size. More particularly, the present invention relates to a process for producing an ethylene polymer having a uniform particle size and a high bulk density.
2. Description of the Prior Art
In recent years, there has been an increasing demand for so-called granular ethylene polymers i.e. ethylene polymers having a relatively large particle size, which can directly be molded by various molding machines without necessity of subjecting the ethylene polymer powder preliminarily to a granulating machine which requires a great deal of energy. Namely, granular ethylene polymers having good granularity and a narrow particle size distribution are suitable for use directly in a molding operation such as injection molding, extrusion molding or blow molding.
Further, it is desired to reduce the installation costs and the production costs by simplifying the process. For this purpose, it is necessary to improve the catalyst system to increase the catalytic activity per the transition metal or per the solid catalyst component, and when such a catalyst is used for a continuous slurry polymerization, it is necessary that the operation can be continued with high efficiency for a long period of time. In this respect, it is necessary to minimize the formation of fine polymer particles and to obtain a polymer having a uniform particle size and a high bulk density, whereby it is possible to increase the concentration of the slurry, to reduce the amount of the solvent required and thus to further reduce the installation and production costs.
The present inventors made extensive researches on catalysts useful for the polymerization, and invented processes in which a catalyst system is employed which comprises an organic metal compound and a solid catalyst component obtained by reacting a halogen-containing titanium or vanadium compound with a reaction product of a specific organic silicone compound with an organic magnesium compound, the solid catalyst being insoluble in an inert hydrocarbon solvent. Japanese patent applications were filed for the inventions and published under Japanese Patent Publication Nos. 19308/75, 19309/75, 154/76 and 12232/77. Further, the present inventors have proposed a process for producing an ethylene polymer having an improved molecular weight distribution, in which a solid catalyst component is employed which is obtainable by carrying out the above-mentioned reaction for the preparation of the solid catalyst component in the presence of an aluminum alkoxide compound or a reaction product of an aluminum alkoxide compound with water (Japanese Laid-Open Patent Application No. 47783/79).
However, the average particle size of the polymer obtainable by the above process is relatively small, and the process is not suitable to obtain a granular polymer having an average particle size of at least 300.mu., preferably at least 500.mu.. Further, in the process, there is a certain room for improvement in the particle size distribution of the polymer. Particularly, it is desired to minimize fine particles which tend to cause troubles such as scattering during the transportation of the powder or which tend to reduce the efficiency of the granulating machine.